User operated drug delivery devices are as such known in the prior art. They are typically applicable in circumstances, in which persons without formal medical training, i.e., patients, need to administer an accurate and predefined dose of a medicinal product, such as heparin or insulin. In particular, such devices have application, where a medicinal product is administered on a regular or irregular basis over a short term or long-term period.
In order to accommodate with these demands, drug delivery devices have to fulfil a number of requirements. First of all, the device must be robust in construction, yet easy to use in terms of handling and in understanding by the user of its operation and the delivery of the required dose or medicament. Further, the dose setting must be easy and unambiguous. Where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose. Moreover, the device should be suitable for recycling. To meet these requirements, the number of parts required to assemble the device and the number of material types the device is made from need to be kept to a minimum.
The medicinal product to be dispensed by means of the drug delivery device is typically provided in a disposable or replaceable cartridge, such as a vial, ampoule or carpule comprising a piston slidably disposed in the barrel of the cartridge. The piston is to be operably engaged with a piston rod or drive ram of the drug delivery device's drive mechanism. By applying thrust to the cartridge's piston in distal direction—that is towards the patient in the present context—a predefined dose of the liquid drug can be dispensed and expelled from the cartridge.
Cartridges as they are typically used with drug delivery devices, such as pen-type injectors are typically sealed by means of a septum. Such a septum is commonly designed as rubber stopper providing an air-tight seal but being pierceable by piercing elements such as needles or cannulae.
A well-known cartridge holder assembly 10 is illustrated in cross section in FIG. 1. This cartridge holder assembly 10 for a drug delivery device comprises a cartridge holder 14 adapted to receive a cartridge 12, which is hermetically sealed with a flexible and deformable septum 22. At its lower and distal end section, the cartridge holder 14 is threadedly engaged with a needle mount 16. Said mount or needle holder 16 comprises a threaded cylindrical portion allowing to screw the needle holder 16 on the threaded neck portion of the cartridge holder 14. At its lower and distal end section, the mount 16 comprises a flange-like bottom face 17, which in a concentrically inner section holds the injection needle or cannula 20.
During an assembly procedure of the needle holder 16, its proximally located needle 20 penetrates the septum 22 with a tipped end. In this way, a fluid-transferring connection for the purpose of dose dispensing can be established. Additionally, the distal and free end of the needle 20 can be provided with a replaceable needle cap 19. Also, the entire cartridge holder assembly 10 can be covered and protected by a protective cap 18.
Depending on manufacturing tolerances and the mutually corresponding design of cartridge 12 and cartridge holder 14, an axial gap 24 of variable size is typically formed between the bottom portion 17 of the needle holder 16 and the distal end face of the cartridge 16. Axial size of this free space area 24 may vary, e.g. due to manufacturing and assembly tolerances. In particular, during dispensing of a dose of the medicinal fluid contained in the cartridge 12, a respective fluid pressure is built-up, which, due to the inherent elasticity of the septum 22, leads to a respective axial expansion of the septum 22. As a consequence, the septum 22 may almost entirely fill said free space area 24 during a dispending procedure.
Due to its elastic properties the septum 22 will store elastic energy during dose dispensing. But as soon as the fluid pressure returns to an initial value after a dose dispensing procedure, the septum 22 relaxes into its initial configuration, which is accompanied by a retraction of the expanded section of the septum 22 back into the cartridge 12. However, also such a retracting motion may in turn lead to a built-up of a non-negligible fluid pressure enhancement and, as a consequence, a certain amount of medicinal fluid may be supplementally expelled from the cartridge 12, which can be typically observed in the form of post-dispending droplet formation at the distal tip of the needle 20.
It is therefore an object of the present invention, to provide an improved cartridge holder assembly for a drug delivery device, which counteracts generation of droplets after termination of a dose dispensing procedure. It is a further object, to provide an effective means adapted to prevent septum deformation during dose dispensing. Furthermore, the invention focuses on an inexpensive as well as on a stable and robust design of a drug delivery device.